The present invention is directed to the field of materials processing using lasers and, more particularly, to a method and apparatus for monitoring in-process laser weld quality via plasma light intensity measurements.
High power lasers are commonly used for materials working processes such as laser welding, cutting, drilling and heat treating. These processes provide a number of advantages over conventional welding processes including speed, consistency and weld quality.
During laser materials working processes, the laser beam is directed to impinge onto a workpiece, which becomes heated and eventually melts and then vaporizes. This vapor and the surrounding gases are ionized by the extreme heat and form a plasma plume between the laser and the workpiece. The plasma can be controlled by shield gas flow. Weld quality is affected by the instability of the plasma formation and by instabilities in process operating conditions such as fluctuations in the laser beam power and shield gas flow, and by workpiece defects such as weld zone contamination and physical deformation.
As the use of laser materials working processes increases in industry, the need for accurate in-process techniques for monitoring process quality also increases. In-process techniques provide important advantages over post-processing, non-destructive quality control techniques such as x-ray and ultrasonic analysis, and visual inspection, and over destructive quality control techniques such as metallography. Post-processing techniques are labor intensive and tedious, and do not enable real time monitoring and control of laser processing.
U.S. Pat. No. 5,304,774 discloses a laser welding monitoring method by use of a narrow-band filter to match an atomic emission line in the plasma. The measured light intensity is used to determine the penetration depth through a correlation of the light intensity to the cross-sectional area of the weld. A narrow-band fiber is not desirable because it reduces the transmitted light intensity be several orders of magnitudes. Furthermore, a poor penetration may not necessary yield lower plasma emission intensity as disclosed in the patent.
U.S. Pat. No. 5,272,312 teaches a laser monitoring method by simultaneously monitoring the infrared emission and UV emission from a workpiece surface. The angle of the optical axes of the UV detector is stated to be preferably at approximately 30.degree. from the weld surface.
These known in-process techniques for monitoring laser processes are not fully satisfactory. Known techniques can falsely reject good parts (type I error) or fail to reject bad parts (type II error). Type I errors result in increased economic costs. Type II errors can be especially important in laser welding processes which form critical welds. The failure to detect defective critical welds can result in potentially defective parts being used in the final assemblies.
Thus, there is a need for a method of monitoring laser materials working processes that can (i) be performed in real time for in-process control; (ii) can accurately distinguish between good and bad welds and reduce Type I and Type II errors; and (iii) can be used to monitor various laser material processes.